Hard to see this as anything but major: HBO announced this morning that it will be making many of its classic shows, including "The Sopranos," "Six Feet Under" and "The Wire" available for streaming with an Amazon Prime subscription beginning May 21. Newer series like "Girls" and "Veep" will follow on a three-year delay. (Although the press release says the programs will be "exclusive" to Amazon Prime, they will continue to be available through HBO Go.)

As is invariably the case, there are some caveats: "Sex and the City" and "Curb Your Enthusiasm" are currently licensed to other services, and HBO's most popular show, "Game of Thrones," is notably absent from the deal. But this is still a major shift for the channel, which has generally treated its past shows as blue-chip assets. The most recent season of "Game of Thrones" retails on Blu-ray for nearly $80; "Mad Men's" most recent is two-thirds that.

How this will affect HBO's relationship with cable providers remains to be see, although the absence of new programming from the Amazon Prime means you can't cut the cord without being several years behind on the cultural conversation -- or at least as long as it takes for the Sunday-night HBO Go backlog to clear. As Time's James Poniewozik points out, the likely motivation is HBO's desire to stay important to a generation of "cord-nevers" for whom, as Anne Helen Petersen wrote last year, a show that's not available via streaming might as well not exist.

I’ll leave it to others to analyze the business implications of this move, which, from where I’m sitting, boil down at least partly to Prime's quickly ramping-up ambitions, partly to "The enemy of Netflix is my friend." (Both Amazon and HBO are in direct competition with the streaming giant.) But at least one of the big motivations for HBO could be cultural: making sure that its legacy, and its brand, are not lost in the emerging canon of binge-watched TV.

Although the terms of the deal were not disclosed -- the Los Angeles Times' Joe Flint has the most thorough overview of what is known -- the other factor is that Amazon, which recently raised the annual price of Prime from $79 to $99, probably paid HBO a boatload of money for the rights. Since Prime's entire streaming operation is a loss-leader, the real purpose being to get customers into the habit of buying physical product from Amazon more frequently, they can afford to spend, especially since even Prime's increased cost is less than the new subscription rates Netflix has just announced.